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<title>Mutual exclusion and synchronization</title>
<section>
<title>Introduction</title>
<para>The HelenOS operating system is designed to make use of the
parallelism offered by the hardware and to exploit concurrency of both the
kernel and userspace tasks. This is achieved through multiprocessor
support and several levels of multiprogramming such as multitasking,
multithreading and also through userspace pseudo threads. However, such a
highly concurrent environment needs safe and efficient ways to handle
mutual exclusion and synchronization of many execution flows.</para>
</section>
<section>
<title>Active kernel primitives</title>
<section>
<title>Spinlocks</title>
<para>The basic mutual exclusion primitive is the spinlock. The spinlock
implements active waiting for the availability of a memory lock (i.e.
simple variable) in a multiprocessor-safe manner. This safety is
achieved through the use of a specialized, architecture-dependent,
atomic test-and-set operation which either locks the spinlock (i.e. sets
the variable) or, provided that it is already locked, leaves it
unaltered. In any case, the test-and-set operation returns a value, thus
signalling either success (i.e. zero return value) or failure (i.e.
non-zero value) in acquiring the lock. Note that this makes a
fundamental difference between the naive algorithm that doesn't use the
atomic operation and the spinlock algortihm. While the naive algorithm
is prone to race conditions on SMP configurations and thus is completely
SMP-unsafe, the spinlock algorithm eliminates the possibility of race
conditions and is suitable for mutual exclusion use.</para>
<para>The semantics of the test-and-set operation is that the spinlock
remains unavailable until this operation called on the respective
spinlock returns zero. HelenOS builds two functions on top of the
test-and-set operation. The first function is the unconditional attempt
to acquire the spinlock and is called
<emphasis>spinlock_lock</emphasis>. It simply loops until the
test-and-set returns a zero value. The other function,
<emphasis>spinlock_trylock</emphasis>, is the conditional lock operation
and calls the test-and-set only once to find out whether it managed to
acquire the spinlock or not. The conditional operation is useful in
situations in which an algorithm cannot acquire more spinlocks in the
proper order and a deadlock cannot be avoided. In such a case, the
algorithm would detect the danger and instead of possibly deadlocking
the system it would simply release some spinlocks it already holds and
retry the whole operation with the hope that it will succeed next time.
The unlock function, <emphasis>spinlock_unlock</emphasis>, is quite easy
- it merely clears the spinlock variable.</para>
<para>Nevertheless, there is a special issue related to hardware
optimizations that modern processors implement. Particularly problematic
is the out-of-order execution of instructions within the critical
section protected by a spinlock. The processors are always
self-consistent so that they can carry out speculatively executed
instructions in the right order with regard to dependencies among those
instructions. However, the dependency between instructions inside the
critical section and those that implement locking and unlocking of the
respective spinlock is not implicit on some processor architectures. As
a result, the processor needs to be explicitly told about each
occurrence of such a dependency. Therefore, HelenOS adds
architecture-specific hooks to all <emphasis>spinlock_lock</emphasis>,
<emphasis>spinlock_trylock</emphasis> and
<emphasis>spinlock_unlock</emphasis> functions to prevent the
instructions inside the critical section from permeating out. On some
architectures, these hooks can be void because the dependencies are
implicitly there because of the special properties of locking and
unlocking instructions. However, other architectures need to instrument
these hooks with different memory barriers, depending on what operations
could permeate out.</para>
<para>Spinlocks have one significant drawback: when held for longer time
periods, they harm both parallelism and concurrency. The processor
executing <emphasis>spinlock_lock</emphasis> does not do any fruitful
work and is effectively halted until it can grab the lock and proceed.
Similarily, other execution flows cannot execute on the processor that
holds the spinlock because the kernel disables preemption on that
processor when a spinlock is held. The reason behind disabling
preemption is priority inversion problem avoidance. For the same reason,
threads are strongly discouraged from sleeping when they hold a
spinlock.</para>
<para>To summarize, spinlocks represent very simple and essential mutual
exclusion primitive for SMP systems. On the other hand, spinlocks scale
poorly because of the active loop they are based on. Therefore,
spinlocks are used in HelenOS only for short-time mutual exclusion and
in cases where the mutual exclusion is required out of thread context.
Lastly, spinlocks are used in the construction of passive
synchronization primitives.</para>
</section>
</section>
<section>
<title>Passive kernel synchronization</title>
<section>
<title>Wait queues</title>
<para>A wait queue is the basic passive synchronization primitive on
which all other passive synchronization primitives are built. Simply
put, it allows a thread to sleep until an event associated with the
particular wait queue occurs. Multiple threads are notified about
incoming events in a first come, first served fashion. Moreover, should
the event come before any thread waits for it, it is recorded in the
wait queue as a missed wakeup and later forwarded to the first thread
that decides to wait in the queue. The inner structures of the wait
queue are protected by a spinlock.</para>
<para>The thread that wants to wait for a wait queue event uses the
<emphasis>waitq_sleep_timeout</emphasis> function. The algorithm then
checks the wait queue's counter of missed wakeups and if there are any
missed wakeups, the call returns immediately. The call also returns
immediately if only a conditional wait was requested. Otherwise the
thread is enqueued in the wait queue's list of sleeping threads and its
state is changed to <emphasis>Sleeping</emphasis>. It then sleeps until
one of the following events happens:</para>
<orderedlist>
<listitem>
<para>another thread calls <emphasis>waitq_wakeup</emphasis> and the
thread is the first thread in the wait queue's list of sleeping
threads;</para>
</listitem>
<listitem>
<para>another thread calls
<emphasis>waitq_interrupt_sleep</emphasis> on the sleeping
thread;</para>
</listitem>
<listitem>
<para>the sleep times out provided that none of the previous
occurred within a specified time limit; the limit can be
infinity.</para>
</listitem>
</orderedlist>
<para>All five possibilities (immediate return on success, immediate
return on failure, wakeup after sleep, interruption and timeout) are
distinguishable by the return value of
<emphasis>waitq_sleep_timeout</emphasis>. Being able to interrupt a
sleeping thread is essential for externally initiated thread
termination. The ability to wait only for a certain amount of time is
used, for instance, to passively delay thread execution by several
microseconds or even seconds in <emphasis>thread_sleep</emphasis>
function. Due to the fact that all other passive kernel synchronization
primitives are based on wait queues, they also have the option of being
interrutped and, more importantly, can timeout. All of them also
implement the conditional operation. Furthemore, this very fundamental
interface reaches up to the implementation of futexes - userspace
synchronization primitive, which makes it possible for a userspace
thread to request a synchronization operation with a timeout or a
conditional operation.</para>
<para>From the description above, it should be apparent, that when a
sleeping thread is woken by <emphasis>waitq_wakeup</emphasis> or when
<emphasis>waitq_sleep_timeout</emphasis> succeeds immediately, the
thread can be sure that the event has occurred. The thread need not and
should not verify this fact. This approach is called direct hand-off and
is characteristic for all passive HelenOS synchronization primitives,
with the exception as described below.</para>
</section>
<section>
<title>Semaphores</title>
<para>The interesting point about wait queues is that the number of
missed wakeups is equal to the number of threads that will not block in
<emphasis>watiq_sleep_timeout</emphasis> and would immediately succeed
instead. On the other hand, semaphores are synchronization primitives
that will let predefined amount of threads into their critical section
and block any other threads above this count. However, these two cases
are exactly the same. Semaphores in HelenOS are therefore implemented as
wait queues with a single semantic change: their wait queue is
initialized to have so many missed wakeups as is the number of threads
that the semphore intends to let into its critical section
simultaneously.</para>
<para>In the semaphore language, the wait queue operation
<emphasis>waitq_sleep_timeout</emphasis> corresponds to
<emphasis><emphasis>semaphore</emphasis> down</emphasis> operation,
represented by the function <emphasis>semaphore_down_timeout</emphasis>
and by way of similitude the wait queue operation waitq_wakeup
corresponds to semaphore <emphasis>up</emphasis> operation, represented
by the function <emphasis>sempafore_up</emphasis>. The conditional down
operation is called <emphasis>semaphore_trydown</emphasis>.</para>
</section>
<section>
<title>Mutexes</title>
<para>Mutexes are sometimes referred to as binary sempahores. That means
that mutexes are like semaphores that allow only one thread in its
critical section. Indeed, mutexes in HelenOS are implemented exactly in
this way: they are built on top of semaphores. From another point of
view, they can be viewed as spinlocks without busy waiting. Their
semaphore heritage provides good basics for both conditional operation
and operation with timeout. The locking operation is called
<emphasis>mutex_lock</emphasis>, the conditional locking operation is
called <emphasis>mutex_trylock</emphasis> and the unlocking operation is
called <emphasis>mutex_unlock</emphasis>.</para>
</section>
<section>
<title>Reader/writer locks</title>
<para>Reader/writer locks, or rwlocks, are by far the most complicated
synchronization primitive within the kernel. The goal of these locks is
to improve concurrency of applications, in which threads need to
synchronize access to a shared resource, and that access can be
partitioned into a read-only mode and a write mode. Reader/writer locks
should make it possible for several, possibly many, readers to enter the
critical section, provided that no writer is currently in the critical
section, or to be in the critical section contemporarily. Writers are
allowed to enter the critical section only individually, provided that
no reader is in the critical section already. Applications, in which the
majority of operations can be done in the read-only mode, can benefit
from increased concurrency created by reader/writer locks.</para>
<para>During reader/writer lock construction, a decision should be made
whether readers will be prefered over writers or whether writers will be
prefered over readers in cases when the lock is not currently held and
both a reader and a writer want to gain the lock. Some operating systems
prefer one group over the other, creating thus a possibility for
starving the unprefered group. In the HelenOS operating system, none of
the two groups is prefered. The lock is granted on a first come, first
served basis with the additional note that readers are granted the lock
in the biggest possible batch.</para>
<para>With this policy and the timeout modes of operation, the direct
hand-off becomes much more complicated. For instance, a writer leaving
the critical section must wake up all leading readers in the rwlock's
wait queue or one leading writer or no-one if no thread is waiting.
Similarily, the last reader leaving the critical section must wakeup the
sleeping writer if there are any sleeping threads left at all. As
another example, if a writer at the beginning of the rwlock's wait queue
times out and the lock is held by at least one reader, the writer which
has timed out must first wake up all readers that follow him in the
queue prior to signalling the timeout itself and giving up.</para>
<para>Due to the issues mentioned in the previous paragraph, the
reader/writer lock imlpementation needs to walk the rwlock wait queue's
list of sleeping threads directly, in order to find out the type of
access that the queueing threads demand. This makes the code difficult
to understand and dependent on the internal implementation of the wait
queue. Nevertheless, it remains unclear to the authors of HelenOS
whether a simpler but equivalently fair solution exists.</para>
<para>The implementation of rwlocks as it has been already put, makes
use of one single wait queue for both readers and writers, thus avoiding
any possibility of starvation. In fact, rwlocks use a mutex rather than
a bare wait queue. This mutex is called <emphasis>exclusive</emphasis>
and is used to synchronize writers. The writer's lock operation,
<emphasis>rwlock_write_lock_timeout</emphasis>, simply tries to acquire
the exclusive mutex. If it succeeds, the writer is granted the rwlock.
However, if the operation fails (e.g. times out), the writer must check
for potential readers at the head of the list of sleeping threads
associated with the mutex's wait queue and then proceed according to the
procedure outlined above.</para>
<para>The exclusive mutex plays an important role in reader
synchronization as well. However, a reader doing the reader's lock
operation, <emphasis>rwlock_read_lock_timeout</emphasis>, may bypass
this mutex when it detects that:</para>
<orderedlist>
<listitem>
<para>there are other readers in the critical section and</para>
</listitem>
<listitem>
<para>there are no sleeping threads waiting for the exclusive
mutex.</para>
</listitem>
</orderedlist>
<para>If both conditions are true, the reader will bypass the mutex,
increment the number of readers in the critical section and then enter
the critical section. Note that if there are any sleeping threads at the
beginning of the wait queue, the first must be a writer. If the
conditions are not fulfilled, the reader normally waits until the
exclusive mutex is granted to it.</para>
</section>
<section>
<title>Condition variables</title>
<para>Condition variables can be used for waiting until a condition
becomes true. In this respect, they are similar to wait queues. But
contrary to wait queues, condition variables have different semantics
that allows events to be lost when there is no thread waiting for them.
In order to support this, condition variables don't use direct hand-off
and operate in a way similar to the example below. A thread waiting for
the condition becoming true does the following:</para>
<para><programlisting language="C"><function>mutex_lock</function>(<varname>mtx</varname>);
while (!<varname>condition</varname>)
<function>condvar_wait_timeout</function>(<varname>cv</varname>, <varname>mtx</varname>);
/* <remark>the condition is true, do something</remark> */
<function>mutex_unlock</function>(<varname>mtx</varname>);</programlisting></para>
<para>A thread that causes the condition become true signals this event
like this:</para>
<para><programlisting><function>mutex_lock</function>(<varname>mtx</varname>);
<varname>condition</varname> = <constant>true</constant>;
<function>condvar_signal</function>(<varname>cv</varname>); /* <remark>condvar_broadcast(cv);</remark> */
<function>mutex_unlock</function>(<varname>mtx</varname>);</programlisting></para>
<para>The wait operation, <emphasis>condvar_wait_timeout</emphasis>,
always puts the calling thread to sleep. The thread then sleeps until
another thread invokes <emphasis>condvar_broadcast</emphasis> on the
same condition variable or until it is woken up by
<emphasis>condvar_signal</emphasis>. The
<emphasis>condvar_signal</emphasis> operation unblocks the first thread
blocking on the condition variable while the
<emphasis>condvar_broadcast</emphasis> operation unblocks all threads
blocking there. If there are no blocking threads, these two operations
have no efect.</para>
<para>Note that the threads must synchronize over a dedicated mutex. To
prevent race condition between <emphasis>condvar_wait_timeout</emphasis>
and <emphasis>condvar_signal</emphasis> or
<emphasis>condvar_broadcast</emphasis>, the mutex is passed to
<emphasis>condvar_wait_timeout</emphasis> which then atomically puts the
calling thread asleep and unlocks the mutex. When the thread eventually
wakes up, <emphasis>condvar_wait</emphasis> regains the mutex and
returns.</para>
<para>Also note, that there is no conditional operation for condition
variables. Such an operation would make no sence since condition
variables are defined to forget events for which there is no waiting
thread and because <emphasis>condvar_wait</emphasis> must always go to
sleep. The operation with timeout is supported as usually.</para>
<para>In HelenOS, condition variables are based on wait queues. As it is
already mentioned above, wait queues remember missed events while
condition variables must not do so. This is reasoned by the fact that
condition variables are designed for scenarios in which an event might
occur very many times without being picked up by any waiting thread. On
the other hand, wait queues would remember any event that had not been
picked up by a call to <emphasis>waitq_sleep_timeout</emphasis>.
Therefore, if wait queues were used directly and without any changes to
implement condition variables, the missed_wakeup counter would hurt
performance of the implementation: the <code>while</code> loop in
<emphasis>condvar_wait_timeout</emphasis> would effectively do busy
waiting until all missed wakeups were discarded.</para>
<para>The requirement on the wait operation to atomically put the caller
to sleep and release the mutex poses an interesting problem on
<emphasis>condvar_wait_timeout</emphasis>. More precisely, the thread
should sleep in the condvar's wait queue prior to releasing the mutex,
but it must not hold the mutex when it is sleeping.</para>
<para>Problems described in the two previous paragraphs are addressed in
HelenOS by dividing the <emphasis>waitq_sleep_timeout</emphasis>
function into three pieces:</para>
<orderedlist>
<listitem>
<para><emphasis>waitq_sleep_prepare</emphasis> prepares the thread
to go to sleep by, among other things, locking the wait
queue;</para>
</listitem>
<listitem>
<para><emphasis>waitq_sleep_timeout_unsafe</emphasis> implements the
core blocking logic;</para>
</listitem>
<listitem>
<para><emphasis>waitq_sleep_finish</emphasis> performs cleanup after
<emphasis>waitq_sleep_timeout_unsafe</emphasis>; after this call,
the wait queue spinlock is guaranteed to be unlocked by the
caller</para>
</listitem>
</orderedlist>
<para>The stock <emphasis>waitq_sleep_timeout</emphasis> is then a mere
wrapper that calls these three functions. It is provided for convenience
in cases where the caller doesn't require such a low level control.
However, the implementation of <emphasis>condvar_wait_timeout</emphasis>
does need this finer-grained control because it has to interleave calls
to these functions by other actions. It carries its operations out in
the following order:</para>
<orderedlist>
<listitem>
<para>calls <emphasis>waitq_sleep_prepare</emphasis> in order to
lock the condition variable's wait queue,</para>
</listitem>
<listitem>
<para>releases the mutex,</para>
</listitem>
<listitem>
<para>clears the counter of missed wakeups,</para>
</listitem>
<listitem>
<para>calls <emphasis>waitq_sleep_timeout_unsafe</emphasis>,</para>
</listitem>
<listitem>
<para>retakes the mutex,</para>
</listitem>
<listitem>
<para>calls <emphasis>waitq_sleep_finish</emphasis>.</para>
</listitem>
</orderedlist>
</section>
</section>
<section>
<title>Userspace synchronization</title>
<section>
<title>Futexes</title>
<para></para>
</section>
</section>
</chapter>